Nuts & Volts - September 2005

the software on a system whose three plug-in memory cards stored a total of 12 KB and had cost my employers $4,000 apiece.

In 1979, I finally bought a computer of my own. Right from the start, I’d decided to buy the best home computer available and to stick with it. That’s why I picked the Compucolor II. It left all its contemporaries in the dust, yet today, virtually no one has ever heard of it. I was still using mine in 1987, when I bowed to the inevitable and bought an Amiga 500. In 1992, I upgraded to the Amiga 3000 I still use. As I said, I stick with the best.

BOTTOM UP OR TOP DOWN?

Early home computers were designed from the ground up. Developers introduced new features as fast as they could design them, and customers gobbled them up. You started with a minimal, barely usable machine with integer Basic and scarcely enough memory to run it. You fought the family for use of the TV as a display device and you struggled to get programs to store and load on an audiocassette recorder. Eventually, you’d expand the RAM capacity, buy a floating point Basic, or even add a disk drive. The TV remained the only display option. Dedicated computer monitors were rare, hugely expensive, and monochrome. Besides, the RF output from your computer wouldn’t drive one.

The Compucolor II bucked this trend. For a start, it was designed not by a bunch of enthusiasts working on a shoestring, but by an offshoot of the Intelligent Systems Corp., a commercial computer and monitor company. Rather than adding on to a minimal system, its designers scaled down a professional product. By the standards of the day, its floating-point, graphics-capable Basic was a marvel and its development was already paid for.

The Compucolor’s plastic case had been designed for a 13” portable TV (see Photo 1). Where you’d expect to find the TV’s tuner, there was a 5- 1/4” floppy disk drive. Down the other side of the CRT was the chassis that held the power supply and the line and frame scan circuitry. Under the tube, you’d find the computer board

SEPTEMBER 2005

with an 8080 processor and from 8K to 32K of RAM. Its 16K of ROM held the disk operating system and Basic.

RAM could be extended to 40K with an add-on board. The advantage to an add-on was that neither Basic nor the operating system was aware of its existence. It was the perfect place to run your own assembly language programs without fear that they might be overwritten. Photo 2 shows the motherboard with two add-on memory boards — one static and one dynamic.

At a time when most home computers had rubber membrane keyboards that were cumbersome and inconvenient, the Compucolor had a real keyboard with real keys. It was a standard Intecolor part, a solidly built unit linked to the computer with a ribbon cable about four feet long. You no longer needed to perch the TV on top of the computer and sit with your nose a foot from the screen. Basic keywords could be entered with single keystrokes.

128 x 128 resolution. The built-in Basic had a full set of point-and-line plotting functions.

All of the pixels in a block shared the same color. If you wanted control over the colors of individual picture elements, you had to settle for 64x64 blocks of four pixels each.

AND A DISK DRIVE, TOO!

A REAL DISPLAY

Apart from being the only home computer of its generation whose disk drive wasn’t an optional extra, the Compucolor’s distinction was its built-in display tube with direct RGB drive. Not only did you have higher resolution than computers driving TV sets, but you also had eight colors and full saturation, rather than the limited range of pastel shades that the Apple II could manage. I used to show mine off at computer clubs; its dazzling color display blew people away.

The character generator limited you to upper-case-only, but you could display 64 characters on each of 32 lines.

Other computers could only manage 40 characters by 24 lines. Each character had an attribute byte that let you choose any two of the eight colors for a character’s background and foreground. You soon learned which combinations were readable and which ones were not.

Because any character could be replaced by a 2x4 pixel block, you could plot graphs with

The Compucolor’s 5-1/4” disk drive was completely non-standard. It read and wrote data using a TMS5501 timer/port/UART chip in its high-speed test mode. Each disk stored 51,200 bytes as 400 blocks of 128 bytes — probably the lowest disk capacity of any commercially available computer. You doubled it by flipping your disks over and putting another 400 blocks on the back. This procedure worked most of the time, even though disk-drive experts condemned it.

The disk operating system was a masterpiece of simplicity. Each disk had a number of blocks allocated to its directory, as selected by the user; each block addressed six files. Each file occupied a contiguous area of the disk. If you edited and saved a file, it was automatically assigned a new version number and appended to the disk and the directory.